This invention relates to integrated circuit (IC) and system modularity and more specifically to the process and the means for implementing the process. More specifically, this invention is related to the application for the process and means of IC modularity to controller based units and particularly to hand held or desk model electronic calculators. Heretofore, the predominant methodology and means for implementing calculator system design required designing custom or semi-custom single chip or multi-chip integrated circuit calculator systems. The design goals were determined by looking primarily to the integrated circuit chip sizes (which is directly proportional to its cost) and to electrical and performance data. A particular calculator integrated circuit or plurality of integrated circuits comprising a calculator system, was thus limited to a particular calculator or line of calculator products. However, each line of calculator products would typically contain different instruction sets, different I/O configurations, different packaging pinouts, and substantial re-layout with re-design would typically be required to increase memory size, or to change the I/O structure, or to modify the pinout. Furthermore, even within a particular calculator integrated circuit system for a particular line of calculators, I/O changes, pinout changes, special functions, and in some cases memory size changes required substantial integrated circuit bar relayout and redesign. In some systems, additional read-only memory and read/write memory was added by means of designing separate integrated circuit chips with an I/O interface structure compatible to the controller to increase the memory capacity of the calculator system. For example, as disclosed in U.S. Pat. No. 4,153,937, assigned to the assignee of the present invention, a series of Texas Instruments programmable calculators (TI-57, 58, 59) are described which are designed so that limited amounts of additional read-only memory and read/write memory may be added to the calculator system to increase its capability and to change the functions which the calculator system performs. However, separate memory busses and separate communications protocol between the base controller chip set and the read-only memory and between the base controller chip set and the read/write memory limits the system's modularity. Furthermore, the printer (external peripheral I/O) interface of this system utilizes yet another communications protocol. These protocols are necessary for compatible interface to the basic timing and control signals as provided by the base controller chip set. The integrated circuits comprising the base controller chip set are themselves fixed in design and adapted to perform the specialized functions of a programmable hand-held calculator. The amounts of read-only memory and read/write memory in the base controller chip set is fixed, and thus does not readily adapt to multiple applications. Furthermore, the pecularities of the architecture, timing, and control signals of a given calculator system, for example, the programmable calculator system described above, do not lend themselves well to a modular system design, as interface integrated circuits and peripherals are designed to be compatible with the base controller chip set which itself requires specialized protocol. Therefore, these peripheral integrated circuits and peripheral I/O devices become specialized in function and are not usable with other base controller chip sets of other calculator systems. The cost and cycle time of these types of designs is becoming prohibitive due to the rapidly changing needs of the market place and the difficulty of achieving an adequate return on investment.